Several methods to record data on magnetic tape are known. Among those are helical scan, arcuate scan, and linear recording. The present invention relates to linear recording.
A typical tape drive for linear recording is shown in FIG. 1. It comprises of a supply reel 2 and a take up reel 3. One or both reels may be enclosed in a cartridge. The tape 6 is moved under the control of a servo system (not shown) between the supply reel and the take up reel past a read-write head 10. A plurality of bearings 4 may be used to guide the tape.
Linear recording devices write the data in individual data tracks parallel to the edge of the recording media. FIG. 2 depicts a typical configuration. Recording media 6 is moved in direction of arrow D1 across the surface of a read/write head 10. The head contains several write elements 11, each of which writes one data track 12 on magnetic tape 6. The head also contains a number of read elements 13. Typically, there are one or two read elements for each write element.
When retrieval of previously recorded data from tape is desired, the media is moved again across the head and the read elements 13 receive the magnetic signal from the media and generate electrical signals that can be decoded by appropriate circuitry. The read elements are also used during a recording operation to ensure that data has been recorded correctly. After the media has moved past the write elements 11 and data has been recorded, the media moves past the read elements 13. The data of each track is read and compared to the data that was written onto that track. Any recording error is detected and an appropriate error correction procedure can be taken. Recording errors can be caused by media defects, contamination of the media, contamination of the head elements, and by other reasons.
It is desired to increase the recording capacity of the tape. This may be achieved is by increasing the number of recording tracks and by increasing the linear density. Newer tape drives commonly have hundreds of parallel tracks. The example shown in FIG. 2 does not support the high track density of newer tape drives. It is not practical to manufacture a head with several hundred of read and write elements. Tape drive manufacturers have therefore adopted a technology called serpentine recording.
Only a small number of tracks is written when the tape is moved across the head, typically between 4 and 18 tracks. When the end of the tape is reached, the head is moved to a different, not-yet recorded area of the tape and a second set of tracks is written while the tape is moved in the opposite direction. When the beginning of the tape is reached, the head is moved again and the tape reverses direction again. This process is repeated until all areas of the tape have been accessed. On some tape drives the number of passes made before the tape has been completely written can exceed 100. This figure will increase in the future.
FIG. 3a shows an exemplary method of serpentine recording. The head (not shown) is first positioned to the upper edge of the tape. The tape is moved in direction indicated by arrow D1 and the track group 20 is recorded. When the end of tape is reached, the head is moved and track group 30 is recorded, while the tape is moved in the opposite direction as indicated by arrow D2. This process continues until the entire tape has been recorded. FIG. 3a shows track group 50 as the last group to be recorded. Although FIG. 3a shows only 3 groups, there may be a plurality of track groups recorded between track group 30 and track group 50. The number of track groups recorded is preferably an even number. When recording of the last group is completed, the tape is back at the starting point, avoiding the necessity to rewind the tape.
Several variations of serpentine recording are known. For example, the recording may commence by writing a track group in the middle of tape. The next group may be located above or below the first group. The third group may then be located on the opposite side of the middle group than the second group. The fourth group may then be located adjacent to the second group, and so on. The tape will be recorded in this manner from the middle of the tape, in an alternating fashion, towards both edges.
FIG. 3b shows another variation. The elements of the head are spaced further apart so that a row of elements may span, for example, ¼ of the width of the tape. The data are written in four track groups 20, 30, 40, and 50. In the example in FIG. 3b the recording of track group 20 has been completed. Track group 30 is in the process of being recorded, while track groups 40 and 50 have not been recorded yet. In this example four tracks are written simultaneously. In a different embodiment the number of tracks written simultaneously may differ and the distance between the tracks may also differ.
The process of recording track group 30 will now be described. The recording of the other 3 groups follows the same method as the recording of track group 30.
After writing the first set of tracks 31, 32, 33, and 34, while the tape is moved in direction of D1, the head (not shown) is moved by a small increment and tracks 35, 36, 37, and 38 are written while the tape is moved in direction D2. Thereafter the head is moved again and the next set of tracks (not shown) is written in direction D1. This process is repeated until the area between the original tracks is recorded. After this section of the tape has been recorded, the head is moved by a large distance and track group 40 is recorded. Thereafter the head is moved again by a large distance and of track group 50 is recorded.
Independent of the particularity of the implementation of the serpentine recording, there are several disadvantages that are common to all implementations. FIG. 4 shows two positions of the read/write head 10. At position P1 the track group nearest to the upper edge of the tape is recorded or read. The location of the read and write elements is shown by outline 14. At position P2 the track nearest to the lower edge of the tape is recorded or read. The surface of the head must support the tape 6 in either position. The length l of the head must be substantially twice the width w of the tape 6.
FIG. 5 shows a head 60 that has a surface 61 over which the tape (not shown) is moved. A plurality of write elements 62 are grouped closely together. The spacing of the write element matches the intended track spacing on tape. On either side of the write elements is a group of read elements. When tape is moved in direction of arrow D1, read elements 63 are used to verify data written by the write elements. When tape is moved in direction of arrow D2, read elements 64 are used to verify data written by the write elements. The length l of the surface 61 must be sufficient to support the tape independent of which track group is recorded.
The large size of the head is disadvantageous for several reasons. First, the overall size of newer tape drives is getting smaller. A large head poses an obstacle to the miniaturization effort. Second, the large mass of the head limits the operation of a actuator. This will be described in detail below. Third, the large number of passes of the head over the tape increases tape wear and head wear. This will also be described in detail below.
The narrow track width of newer tape drives requires the head to be positioned precisely at the intended position. Manufacturing tolerances of the head, combined with inaccuracies of the tape and fluctuations of the tape movement across the head, make this precision difficult to achieve. Most newer tape drives, therefore, adopt a technique called “track following” to overcome this difficulty. The media manufacturer typically writes one or several highly precise servo tracks onto the tape. Special elements in the head are used to decode the servo tracks when the tape is recorded or read in a tape drive. The information from the servo tracks is used to obtain the relative position of the head to the media. A head actuator is then used to move the head to the correct position. The position of the head is adjusted constantly as the tape is moved over the head. Several methods of track-following systems are known. The closed-loop systems help to maintain the proper head-to-tape alignment. However, the mass of the head limits the frequency response of the system. This limitation is undesirable since further decreases in the track width cannot be achieved without a fast track-following system. A read/write head that has a low mass is therefore desirable.
The current method where tape flies over the head also poses a formidable obstacle when attempting to increase the linear density of tape.
In order to increase the linear density of the recording the tape has to be in close proximity of the read and write elements. When the tape moves over the recording surface of the head air gets trapped between the head surface and the tape. This air film separates the tape from the head. This self-lubrication of the air film is desirable since it reduces tape wear. However, as the recording densities increase, the separation between the tape and the head must be decreased. Several parameters influence the flying height of the tape over the head, including the tape speed, tape tension and the head contour. It is desirable to increase the recording speed of the tape in order to decrease the recording time. The increased tape speed will increase the flying height of the tape.
While the tape is moved over the head, the tape is held under a controlled tension. The tension used is generally proportional to the thickness of the tape. It is desirable to decrease the tape thickness in order to increase the tape length and the recording capacity. The increasingly thinner tapes are wound with lower tensions. The lower tension increases the flying height of the tape over the head. Therefore a head that allows a low flying height at a high tape speed and at a low tape tension is therefore most desirable.
An additional problem with the current method is increased tape wear and contamination of the head as a result of the tape wear. When tape moves over the head an air film between the tape and the head may separate the tape from the head. However when tape movement is stopped, the air film dissipates and the still-moving tape contacts the head. The friction between the tape and the head causes wear of the tape and the head. This wear limits the lifetime of both the tape and the head. In addition, loose particles of debris collect on the head and on the tape, resulting in read and write errors.
Referring now again to FIG. 4, we can see that current methods are causing excessive tape wear. For example, when the head 10 is in position P2, track group 50 at the lower edge of tape is recorded and verified. Head 10 spans over all previously recorded data groups. However, the previously recorded data cannot be verified again when the tape moves over the head to record track group 50. The number of tracks recorded on tape is increasing faster than the number of tracks recorded in parallel. This leads to an increase in the number of times the tape passes over the read-write head before the entire tape is recorded. The high number of passes, combined with the lack of verification, poses a serious risk to the reliability of the data.
It is now clear that an improved read/write head is desired.
It is an object of the present invention to reduce the size of the read/write head to allow miniaturization of the tape drive.
It is another object of the present invention to reduce the mass of the head to improve the frequency response of the track-following system.
It is another object of the invention to control the separation between the head and the tape.
It is another object of the invention to reduce the tape and the head wear.
It is another object of the invention to reduce the tape and the head contamination.